A woman was arrested for investigation of attempted murder after she was found at a post office with more than 100 envelopes containing bags of deadly sodium cyanide disguised as nutritional supplements.

Kathryn Schoonover was arrested Sunday afternoon at a post office in Marina del Rey, about 10 miles southwest of Los Angeles, Los Angeles County Sheriff Sherman Block said."Had this stuff gone into the mail, we would have had mass murder," Block said.

The envelopes were addressed to people across Southern California and as far away as New York. Each business-size envelope contained a clear plastic bag filled with about a teaspoon of the cyanide in powder form, and each bag was attached to a brand-name brochure that claimed the powdery substance was a nutritional supplement, authorities said.

The poison, which interrupts the uptake of oxygen in cells, causes almost instant death if consumed.

"This is essentially the same chemical they use in the gas chamber," said Frank Cervantes of the Los Angeles County fire department.

Investigators were interviewing Schoonover, 50, to determine a motive.

Block said no envelopes had been mailed at the Marina del Rey post office, but there was some concern she may have mailed envelopes elsewhere along the coast, so the U.S. Postal Service was checking mailboxes along 90 miles of coast between Los Angeles and Schoonover's hometown in Carpinteria, a coastal town southeast of Santa Barbara.

Deputies also planned to contact the addressees. Block said there did not appear to be any pattern to the mailings.

The envelopes were discovered after a passer-by in the post office became suspicious when she saw Schoon-over stuffing the bags into envelopes and saw a container on the counter with poison symbols. The passer-by contacted authorities.